It always helps to rake the lawn to agitate the soil and thatch, and keep the lawn moist, water every day if you want it to take... never let it dry out until it sprouts. Some will grow if you just scatter seed, but mother nature doesn't always give you the ideal conditions.

No, yes, and yes. Just sprinkling it on the ground will get you next to nothing. Rack or loosen the dirt or mix it with dirt then spread the grass seed. You can tamp it down with your feet or a roller to prevent it from washing away. KEEP IT WET AT ALL TIMES. If it dries out even once chances are it won't take.

Just sprinkling the seed over an existing lawn will get some result. 99% of the seed will likely not germinate and take root. But since you'll be putting down millions of seeds there will be a good number of new grass plants. Depending on the condition of your lawn you may or may not notice.

Using a pulverizer (a tractor powered implement to super rake/scratch the soil) will dramatically improve germination. So much so that you'll easily see the new grass sprouting in rows like you'd planted a field. If you are doing your whole lawn some type of powered tool or machine is a very good idea. Even with a small lawn you'll quickly tire of the thorough raking required to get through the grass and make significant scratches in the earth.

For a small repair patch or a micro sized lawn hand raking is an option. You need to use a hard tine garden rake and really work it. You will rake up a lot of thatch in the process so have an empty garbage can or bag ready for the waste. Remember that your task is not removing the thatch (which is good for most lawns) but are trying to get through it to rake up/disturb the soil below. A light raking after spreading your seed can help knock the seed down to ground level and get it in contact with the soil.